The corrosion of industrial piping and other components such as valves and pumps is a major problem in some industries. The oil industry, in particular, faces severely corrosive environments, with corrosive gasses and liquids such as H2S (hydrogen sulfide) at elevated temperatures and pressures. Additionally, these conditions form severe wear and erosion environments. One solution to these issues is to coat a lower grade base material with a high quality coating material having the desired high corrosion and wear-resistant properties. Typically, these types of properties will be found in metal, ceramic and particularly diamond-like carbon coatings.
Expensive specialty alloys, such as Hastelloy and Inconel (both of which are federally registered trademarks of Huntington Alloys Corporation), are commonly used for exhaust piping in the chemical processing industries. These alloys exhibit high temperature strength and corrosion resistance. Again, a less expensive base material can be used if a suitable surface coating is applied to the interior surface that is to be exposed to the corrosive environment.
Prior art coating methods include chemical vapor deposition (CVD), physical vapor deposition (PVD), plasma spray, electroplating and sol-gel. Of these methods, CVD and PVD provide the highest quality films with regard to purity, adhesion, uniformity and other properties. Both of these techniques require the use of a specialized vacuum chamber, making it difficult to coat fully assembled components. In the case of applications using piping, valves, pumps or tubing for carrying corrosive material, such as the oil/petrochemical industry, the interior surface that is in contact with the corrosive material must be coated. For very low pressure techniques such as PVD, where the pressure is below or near the molecular flow region, coating interior surfaces has been limited to only large diameter and short length (small aspect ratio) tubes. Similarly, CVD techniques are limited to such applications, due to the need to supply heat for the chemical reaction, which can damage heat-sensitive substrates. Plasma enhanced CVD (PECVD) can be used to lower the temperature required for reaction, but there is then difficulty in maintaining uniform plasma inside the pipe and in preventing depletion of source gas as it flows down the pipe.
The plasma immersion ion implantation and deposition (PIIID) technique has been shown to be useful for coating the external surfaces of complex shapes. PIIID is performed by applying a negative bias to the workpiece, which will pull positive ions toward the workpiece, if the plasma sheath is conformal. There are also improvements that can be made to film properties such as adhesion and film density via ion bombardment of the workpiece.
In prior art PVD or CVD chambers, the chamber dimensions are designed such that there is very little change in pressure throughout the chamber. When using a workpiece as a chamber, one has no control over the shape of the chamber/workpiece and so the process must be designed to account for workpieces with high aspect ratios (length/diameter) in which there is a significant pressure drop from the gas inlet to the exit. This invention provides a method of coating such workpieces with good uniformity. Also it may be desirable to coat the internal surfaces of sections of pipe and then assemble the sections by welding. In this case, it is necessary to coat the welded areas of a large length of pipe. The invention provides this ability.
Methods of coating the interior surface of tubes have been described whereby the source material to be applied is inserted into the tube and then sputtered or arced off onto the tube. For example, U.S. Pat. No. 5,026,466 to Wesemeyer et al. describes a method of inserting a cathode into a tube and arcing the cathodic material onto the inside of the tube. U.S. Pat. No. 4,407,712 to Henshaw et al. describes a hollow cathode with a high evaporation temperature metal source inserted into a tube, with a cathode arc removing the source material from the hollow cathode and coating the inside surface of the tube. This type of arrangement has several drawbacks, including being limited to only large diameter tubes (due to having to insert the hollow cathode tube with associated heat shield and cooling tubes into the tube to be coated), the requirement of complicated arrangements for motion of anode and hollow cathode through the tube, and the generation of macro-particles by cathodic arc. U.S. Pat. No. 4,714,589 to Auwerda et al. describes coating the inside of a tube by plasma activated deposition of a gas mixture, but this method is limited to electrically insulative tubes and coatings, and involves a complicated system for moving a microwave source along the outside of the tube.
In situations in which welded areas or sections of pipe require coating, methods for doing so typically involve an apparatus or structure inserted into the pipe. The structure is then drawn along the pipe to coat a large area or is positioned at the welded area to be coated. For example, U.S. Pat. No. 6,699,324 to Berdin et al. describes a method or vehicle which can travel in a pipe and be pulled along a length of the pipe in a manner which allows a rotating distributor to evenly coat the inner walls of the pipe. While this method works well, other improvements are available to uniformly coat welded areas or sections of a pipe.